iwlwifi: mvm: generate statistics debugfs code

There's no need to duplicate the structure field name in the string,
just generate the string in the macro that's there anyway. To keep
the debugfs output the same, rename one (otherwise unused) field.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>